Crude petroleum is distilled and fractionated into many products such as gasoline, kerosene, jet fuel, asphaltenes, and the like. One portion of the crude petroleum forms the base of lubricating oil base stocks used in, inter alia, the lubricating of internal combustion engines.
The manufacture of lubricating base oils from crude petroleum oil is typically a multi-step process, though there are many variations in the specifics of the processing steps, throughout the industry. Each lube manufacturing facility may include one or more of an upgrading step to remove heteroatoms and to increase the viscosity index of the final lube oil product, a dewaxing step to remove undesirable wax from the oil, and a finishing step to stabilize the oil to oxidation and thermal degradation. However, lube oil users are demanding every increasing base oil quality, and refiners are finding that their available equipment is becoming less and less able to produce base stocks which meet these product specifications. New processes are required to provide refiners with the tools for preparing the modem base oils using existing equipment at lower cost and with safer operation.
Bayle, et al., in U.S. Pat. No. 4,622,129, discloses a method of solvent extraction followed by hydrotreating to produce consistently high quality lube oils. In the '129 method, a formula relating to lube oil properties and hydrotreating conditions is proposed for adjusting the extraction depth of the base oils to be hydroprocessed.
Dun, et al. in U.S. Pat. No. 3,663,422, discloses a process for producing very high-viscosity-index lubricating oils by hydrotreating a solvent-refined asphalt-free waxy hydrocarbon oil in the presence of a sulfided catalyst comprising a Group VI and/or Group VIII metal supported on a substantially non-acidic refractory oxide base. The hydrotreating conditions in '422 include a temperature in the range of from 420 to 460.degree. C. and a pressure of from 165 to 225 kg/cm.sup.2. In the '422 process, the hydrotreating step produces a dewaxed oil having a viscosity index of at least 125 and a viscosity at 210.degree. F. (99.degree. C.) of at least 9 centistokes.
Miller, in U.S. Pat. No. 5,413,695, teaches a process for producing lube oil from a solvent refined gas oil, using an intermediate pore size silicoaluminophosphate molecular sieve and at least one Group VII metal under dewaxing and isocracking conditions.
Shen, in U.S. Pat. No. 4,394,249, discloses a hydrodesulfurization process for removing from 50 to 99.5% by weight of sulfur in a lube oil feedstock prior to dewaxing the feedstock in a distillate dewaxing process unit. In the process, a lube oil feedstock is hydrodesulfurized and the effluent separated into a gaseous fraction and a liquid fraction. The liquid fraction is contacted in a catalytic dewaxing unit with a highly siliceous zeolite ZSM-5 type porous crystalline material, and the effluent conducted to a heat exchanger. The gaseous fraction from the hydrotreater is also conducted to the heat exchanger, heated by exchanging heat with the effluent from the catalytic dewaxing unit and conducted to the catalytic dewaxing unit.
Ackelson, in U.S. Pat. No. 4,695,365, discloses a process for hydrotreating and hydrodewaxing a spindle oil in the presence of a catalyst containing at least 70 percent by weight of an intermediate pore molecule sieve in a support. In the preferred process of '365, substantial amounts of sulfur and nitrogen are removed during the process, but the viscosity of the spindle oil remains largely unchanged. Thus, the viscosity of the spindle oil, measured at 100.degree. C., differs from the feed entering the hydrotreating stage by no more than 1.75 centistokes.
Woodle, in U.S. Pat. No. 3,779,896, teaches a preparation of lubricating oil comprising simultaneous deasphalting-solvent refining of a residuum containing petroleum fraction, and hydrocracking the raffinate phase with a hydrocracking catalyst at a temperature between 600.degree. and 900.degree. F. (316.degree. C. and 482.degree. C.), a pressure between 800 and 5,000 psig (5.6-34.6 MPa), a space velocity between 1.0 and 5.0 v/v/hr. and a hydrogen rate between 500 and 20,000 scf/bbl (i.e., standard cubic feet per barrel) (89.1-3600 std m.sup.3 H.sub.2 /m.sup.3 oil).
API Publication 1509: Engine Oil Licensing and Certification System, "Appendix E-API Base Oil Interchangeability Guidelines for Passenger Car Motor Oil and Diesel Engine Oils" describes base stock categories. A Group II base stock contains greater than or equal to 90 percent saturates and less than or equal to 0.03 percent sulfur and has a viscosity index greater than or equal to 80 and less than 120. A Group III base stock contains greater than or equal to 90 percent saturates and less than or equal to 0.03 percent sulfur and has a viscosity index greater than or equal to 120. In order to prepare such high quality oils from straight run petroleum stock conventionally requires very severe operating conditions, including, for example, reaction over a hydrocracking catalyst at a hydrogen pressure generally above 2000 psia (13.8 MPa) and a reaction temperature above about 725.degree. F. (385.degree. C.), or solvent extraction at high solvent/oil ratios and high extraction temperatures. While effective for preparing the base stocks, these conventional processes are expensive to operate, and yields of base stock are often low. It is desirable to have a process for preparing the Group II and Group III base stocks at lower operating cost, at lower equipment cost and with improved operator safety.